1. The Field of the Invention
The present invention relates generally to data transmissions in a wireless network. More particularly, the present invention relates to a method and apparatus for improving transmissions of a data stream in a voice optimized network.
2. The Relevant Technology
Wireless communication systems have become increasingly more prevalent and have contributed greatly to the dynamic nature of modem society. In general, wireless communication systems enable individuals to maintain communication contact with other users at either fixed site, mobile stations, or both. In the past, however, wireless communication systems were predominantly limited to, and primarily tailored for, voice users, such as wireless users interacting with other users or systems using verbal communication. Thus, in order to improve customer satisfaction, wireless communication systems have been increasingly adapted to favorably facilitate communication beyond that of mere voice communications.
For example, wireless communication systems are additionally used for the transmission of data information such as that used in paging, faxing and other similarly related digital transmission technologies. Yet, until recently, digital transmissions tended simply to accommodate digital data by employing low transmission data rates. Although adequate for small amounts of digital data, low transmission rates are extremely inadequate for the substantially larger amounts of data tending to be transmitted in today's wireless networks. In particular, the transmission of substantial amounts of digital data at low data rates congests communication channels, especially those data transmissions in analog cellular networks where reliable transmissions occur at low data rates around 9600 bits per second.
An additional and increasingly more popular use of wireless data transmission involves wireless access of the Internet. Such access enables a user, having a personal portable computer coupled via a modem to a wireless transceiver, to access vast amounts of digital data from both remote and mobile locations. Because of the large amounts of data typically transferred during such Internet activities, customer-satisfactory modems now require data-transfer rates of at least 28.8 kilobits per second and higher. Accordingly, to facilitate such high data-transfer rates, modems have become increasingly more sophisticated.
The problem, however, is that wireless transceivers, because of their still large aptitude for predominantly communicating in voice transmissions, are not yet fully adapted for data transmissions and cannot yet efficaciously handle such large data-transfer rates. In fact, most wireless transceivers (i.e., cellular phones) are still only able to transmit data if coupled to an additional hardware device such as a wireless modem. Even further compounding the problem is the known characteristic that spectrally defined voice and data streams exhibit radically different spectral profiles, to which, the wireless transceivers spectrally favor the voice over the data.
For example, with reference to FIGS. 1 and 2, it can be seen that the respective spectral amplitude distributions for voice 20 and data 22 streams are dissimilar, especially with respect to the higher frequencies. In particular, the spectral profile of voice stream 20 typically exhibits large amplitude levels at lower frequencies and diminished amplitude levels at higher frequencies. Whereas, the spectral profile for data stream 22, contrastingly, maintains a relatively constant or uniform amplitude level across the entire frequency spectrum.
In this manner, if voice stream 20 were directly modulated in a communication network without undergoing any spectral modifications, noise contamination 24, acquired through the communication channel, for example, would be so close in magnitude to the lower amplitude levels of the higher frequencies that the low amplitude levels would be adversely affected by, and generally indistinguishable from, the noise. Therefore, conventional wireless transceivers 26 (FIG. 2) at the transmitting end of an exemplary wireless communication network 28 typically house a pre-emphasis module 30. The pre-emphasis module 30 boosts the low spectral amplitude levels of the voice stream 20 at the higher frequencies to yield a pre-emphasized voice stream 32 (FIG. 1). By boosting the higher frequencies, the voice information becomes more robust and less susceptible to interference from noise 24 injected when transmitted through a communication channel 34. In other words, if the injected noise floor remains roughly constant across all frequencies, then the boosted amplitude levels of the higher frequencies enables wireless transceiver 36 at the receiving end to extract the higher frequencies out of the communication channel noise floor. Post-processing of pre-emphasized voice stream 32, through a de-emphasis module 38 at the receiving end of the wireless network, is typically employed to reduce the amplitudes of the higher frequencies back to the typical voice information profile similar to that shown in voice stream 20.
A problem with pre-emphasizing the voice stream, however, exists because data stream 22 must also passes through pre-emphasis module 30 of wireless transceiver 26. As a consequence, the data stream amplitude levels at the higher frequencies are also further augmented as shown in the frequency domain, as pre-emphasized data stream 40, and in the time domain, as pre-emphasized data stream 42. Because higher amplitude levels correspondingly cause a spread in the frequency spectrum of an FM signal, as is well known in the art, and because each channel within a wireless network 28 has an assigned frequency bandwidth, a time domain limiter 46 is employed to clip the higher amplitude levels of the data stream when such augmented amplitude levels exceed a certain, pre-determined amplitude threshold 44. This prevents bandwidth interference between neighboring channels. Such clipping, however, adversely affects the integrity of the data as shown in the clipped data information profile 48. Furthermore, clipped data results in non-linear distortion of the original data information which greatly facilitates the introduction of transmission errors.
Consequently, the prior art has attempted to remedy this problem. Although some effective remedies could include hardware and software changes internally within the confines of the wireless transceiver, such remedies are not cost effective because they entail redesign and/or physical manipulation of components therein. In addition, such manipulation often voids consumer warranty protections that might exist. As such, most remedies take place in data communication devices that electrically precede the wireless transceivers at the transmitting end of the wireless communication network. Most frequently, the data communication devices are wireless modems 50, 52. At the transmitting end, wireless modems 50 reduce the amplitude levels of the data stream 22 at the higher frequencies at stages prior to the introduction of the data stream into the wireless transceiver 26 so that when the data stream passes through the pre-emphasis module 30, the higher amplitude levels will not be clipped. In particular, prior art configurations have included within modem 50 a data de-emphasis module 54 to pre-condition the data stream such that the signal spectrum will be approximately flat after passing through pre-emphasis module 30. As a result, by reducing, or de-emphasizing, the amplitude of the higher frequency components of the data stream, the de-emphasized data stream 56 is better able to benefit from the processing of the pre-emphasis module 30 that is resident within the wireless transceiver. Such de-emphasized data stream, when presented to pre-emphasis module 30, becomes again emphasized in the higher frequencies thus giving the appearance of the originally flat-spectrumed data stream as depicted by data profile 58. Thus, the frequency components are less susceptible to the influence of noise when introduced through the communication channel.
An additional problem, however, is that conventional wireless transceivers further comprise a compressor stage 60, often overlooked by engineers designing modem interfaces between computers 61 and wireless transceivers, that precedes the pre-emphasis module 30. Although compressors are well known in the art, it is their electrical sequential positioning within the wireless transceiver that has been greatly overlooked. The compressor positioning, it should be appreciated, is after the data de-emphasis module 54 in modem 50 but before pre-emphasis module 30 in wireless transceiver 26, which is all sequentially before the voice and/or data streams are transmitted across communication channel 34. The problem of overlooking the sequential positioning of modules causes the data stream to enter pre-emphasis module 30 in the wireless transceiver after having been previously compressed whereas, in contrast, the same data stream has previously undergone de-emphasis in de-emphasis module 54 of modem 50 without having undergone compression. Although such sequential positioning would not be as crucial if the compressor were a linear device, it matters greatly because compressors in wireless transceivers are highly non-linear which creates a very different signal upon passage therethrough.
On the receiving end of the wireless network, the problem of sequential ordering is further complicated because the reverse, or complimentary, functions performed to reverse what was performed by the transmitting end, takes place in the wireless transceiver, (i.e., reversing the pre-emphasis, expanding in expander stage 62, and undoing the compression). It should be apparent that no reversal, or complimentary, functions take place in receiving end modem 52, other than typical functions such as de-modulation in demodulator 64 undoing the modulation from modulator 66 performed on the data stream. Whereas, it should be appreciated, the entire de-emphasis of the data stream took place in the data de-emphasis module in the transmitting end modem 50. Thus, system mismatch has occurred which lends itself to the introduction of further errors in the transmission of data through a wireless network. Therefore, systems and methods are needed for improving the transmission of data across a wireless network via a wireless transceiver that is adapted, and optimized, for voice transmissions.